This invention relates to making steel and particularly the use of dolomite lime in making steel. Lime is a well known ingredient to be one of the additives in making steel. It is typically used in making steel by an electric arc furnace (“EAF”), basic oxygen furnace (“BOF”), bottom blow oxygen furnace (“Q-BOP”), and even in ladle metallurgy furnaces (“LMF”).
In a typical operation of an EAF, solid charge ingredients including raw scrap, limestone, burnt lime, iron ore and ferroalloy additives are charged to a furnace. Several handling schemes have been developed for the introduction of dolomitic lime, hical lime and other flux materials into the furnace. These schemes include pneumatic injection, batch loading with mobile equipment, and various forms of top feed. These top feed systems, also called “top charge” units, include that are partially or completely automated and continuously or semi-continuously deliver flux, alloy, and/or carbonaceous materials and introduce the material in the roof or sidewall of the furnace. Such furnaces may be equipped with a roof swing which swings the roof aside when cold scrap is charged; a rocker/rail tilting arrangement which allows the furnace to tilt forward for tapping and backward for slagging; an injection system for supplying additions through the furnace roof; and evacuation system for removing dust generated during the steel-making procedure.
In an EAF, electrodes are typically supported overhead and project downwardly through the furnace roof. The electrodes produce an electric arc between the electrodes and scrap and produce heat, which melts the charge and refines the steel. The EAF has evolved considerably with the use of exothermic reactions to complement the electric arc for scrap melting and steel refining. The exothermic reactions have come in two (2) forms: direct oxygen injection using oxygen lances, and the use of oxy-fuel burners. Each of these energy input systems is more efficient for transferring heat in different parts of the campaign, and each may be controlled to inhibit detrimental interaction between the electrodes, the oxy-fuel burners, and the oxygen lances, as well as to avoid any loss of production or losses of yield efficiency. The molten steel is tapped at about 3,000° F. into a ladle where it may be further refined and cast by ingot casting, continuous casting, or by a thin strip casting process.
Particulate emissions are generated at several points during the steelmaking process. For instance, such emissions are generated during charging of the scrap, during tapping of the furnace, during pneumatic injection of additives, during oxygen blowing, as well as during melt down periods. Customarily in the industry, the EAF dust is collected in baghouses. Because this dust contains heavy metals, the State and Federal environmental regulatory bodies have designated all electric arc dust as hazardous waste. As a result, the disposal of such collected dust presents an ever increasing problem and it has become mandatory to find suitable environmentally accepted methods of dust disposal.
Lime is a recognized costly expense in making steel. During the production of the metallurgical grade dolomitic and hical lime, a significant degree of breakage occurs during calcination. Typically, lime produced in a rotary kiln will include greater than 25% of material sized less than 0.25 inch exiting the kiln. Due to rapid hydration and further breakage during shipping, approximately 25% of lime may arrive at the lime user, such as a steel manufacturer, as a powder. Many of the furnace additive handling schemes described above are inefficient at delivering undersized or powdered materials. Direct losses of undersized material from these material handling systems and losses to the emission control system of the furnace can result in very poor recovery of undersized or powdered materials in the furnace. Undersized material that is lost from the material handling system must be collected and disposed of in an environmentally acceptable fashion. Undersized material that is lost to the furnace emission control system is collected in the bag house. This material is intermingled with heavy metals and must be disposed of as hazardous waste. These losses and waste disposal costs add greatly to the cost of lime and other flux materials containing large amounts of powder or undersized material.
The cost for handling and treating this fine lime waste is significant. The cost of the undersized lime is incurred as part of the original lime delivery that is not utilized in the steelmaking process, and then a second time when the undersized lime is recovered and must be disposed of accordingly. As a result, most steelmakers specify a sizing for the lime that excludes material below a specified sizing limit that is determined by the characteristics of their handling system.
Most handling schemes for introducing flux materials into the steelmaking process are sensitive to the sizing and distribution of material being transported. In most systems, the recovery of undersized material, typically less than about 0.25 inch, is poor. This is due to the fact that lime powder is extremely light-weight, causing it to be lost from transfer points in the steelmaking process and uncontained transport equipment including transfer belts. The fine lime material that escapes the material handling system into the plant facility is also an extreme skin irritant, and is capable of causing mild to moderate skin, eye and respiratory burns.
In order to successfully recycle the fine lime to the furnace, it is generally necessary to bind, agglomerate or encapsulate the fine material in some manner. Direct re-injection of the untreated fine material tends to further pollute the work place. Certain of the prior art processes have attempted to pelletize the fine material in order to enhance its storage and handling characteristics. Although such pellets have been successfully utilized in recycling processes of the type described, they typically involve a chemical bonding or agglomerating reaction which requires a predetermined cure time. Also, the previously known pellets have been difficult to store or have partially disintegrated during transit and reinjection into the furnace.